Single mode microwave device for producing exfoliated graphite

ABSTRACT

A device for producing exfoliated graphite from graphite flakes, intercalated graphite, or expanded graphite by means of microwave heating using single mode microwave cavities, a method of producing such materials and products from such methods.

This application claims priority from U.S. Provisional application Ser.No. 61/629,871, filed Nov. 30, 2011.

BACKGROUND OF THE INVENTION

The present invention is in the technical field of nanotechnology. Moreparticularly, the present invention is in the technical field ofnanomaterial manufacturing. More specifically, the present invention isin the technical field of graphite exfoliation.

Graphite is a well known material occurring in natural and syntheticforms. Graphite is made up of layered sheets of hexagonal arrays ornetwork of sp2-carbon atoms, called graphene. A useful form of graphiteis exfoliated graphite. Exfoliated graphite generally is exfoliated orpartially delaminated graphite having a Braunauer-Emmett-Teller (BET)surface area greater than BET surface area of graphite but less than BETtheoretical surface area of single graphene sheet.

Exfoliation of graphite can be performed by applying heat to graphite bya variety of means. Heating by direct application of heat generallyrequires a significant amount of energy, especially in the case oflarge-scale production. Radiofrequency (RE) or microwave expansionmethods can heat more material in less time at lower cost.

THE INVENTION

In one embodiment, the present invention relates to a microwave devicefor the production of exfoliated graphite and to products produced bysuch a device, thus, there is a single mode microwave device, the singlemode microwave device comprising a stainless steel housing comprised ofa top, a bottom, two side walls, a front, and a moveable back wall, allforming a hollow cavity. The moveable wall has a back surface, andfixedly attached to the back surface is a control rod.

There is contained in the housing an exfoliation tray, the exfoliationtray being supported by at least two support rods which rods aresupported at predetermined levels by the side walls. The exfoliationtrays are electrically non-conducting.

The bottom, front wall and side walls have a cooling jacket mounted onthe outside surfaces and there is an adapter surmounting the top of thehousing. The moveable back wall has at least one gas inlet port throughit and the bottom of the housing contains an exhaust port.

Fingerstock gaskets or conductive brushes are attached to the edges ofthe moveable back wall to retain electrical contact between the moveableback wall and the housing and there is a microwave generator to delivermicrowave energy to the hollow cavity using a rectangular waveguide thatis connected to said adapter. For purposes of this invention,“fingerstock gaskets” and “conductive brushes” are interchangeable.

Fingerstock gaskets are purchasable from www.surplussales.com. A gasketthat works particularly well in this invention is 97-440-02MFingerstock, Cu-Be.

In another embodiment, there is a method of exfoliating graphite, saidmethod comprising providing a nanomaterial and placing the nanomaterialin the hollow cavity of a device as set forth Supra.

Thereafter, moving the moveable back wall of the device such that thenanomaterial is in resonance with the microwave frequency inside thecavity, and heating the cavity until the nanomaterial is exfoliated andthereafter, removing the exfoliated nanomaterial from the hollow cavity.

An additional embodiment is a method for preparing exfoliated graphene,the method comprising using a single mode microwave device as set forthSupra while the single mode microwave device is operating, deliveringintercalated graphite at a controlled rate to the exfoliation traythrough the delivery chute while controlling the microwave frequenciesat a frequency selected from 915 MHz and 2.45 GHz.

Other, additional embodiments are products prepared by the methods asset forth just Supra.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a full view in perspective of a microwave device of thepresent invention.

FIG. 2 is a full cross sectional view of the microwave device of FIG. 1through line A-A.

FIG. 3 is a full side view of a microwave device of FIG. 1.

FIG. 4 is a full cross sectional view of the microwave device of FIG. 3through line B-B.

DETAILED DESCRIPTION OF THE INVENTION

The instant invention provides a device for exfoliating intercalatedgraphite, or expanded graphite by applying microwave energy to theintercalated graphite, or expanded graphite inside a microwave cavity.Such a device is a single mode microwave 1 as shown in FIG. 1.

In FIGS. 1 and 2, the microwave housing 3 is made of stainless steel orof any other metals that exhibit electrical conductivity. The dimensionsof the microwave cavity 2 formed by the housing 3 are calculated basedon the resonance dimension of the operating frequency of the microwaveinside the rectangular cavity. In general, the microwave frequenciesused for industrial application in the U.S. are 915 MHz and 2.45 GHz.

To adjust the cavity 2 dimensions, a movable wall 4 is used. Thismovable wall 4 can be set so that the microwave cavity 2, along with thematerial inside the microwave cavity 2, is in resonance condition withthe microwave frequency. A rod 5 is connected to the movable wall 4 sothat the adjustment of the wall's position relative to the dimension ofthe cavity 2 can be performed outside of the microwave cavity 2. The rod5 may be either solid or hollow. With this sliding wall feature thedimension of the microwave cavity 2 can be adjusted to allow resonancecondition and to produce a specific resonance mode of which its locationis at the intercalated graphite load 6 (See FIG. 2). Fingerstock gaskets7 are attached to the edges 8 of the movable wall 4 to ensure that thewall 4 can be moved while it still electrically connected to the rest ofthe microwave cavity wall.

Microwave energy is delivered from a microwave generator (not shown) tothe microwave cavity 2 using a rectangular waveguide that is connectedto an adapter 10 on the top side 11 of the microwave cavity 2. Toimprove the coupling of the microwave into the cavity, an iris structuremay be used.

The intercalated graphite or the expanded graphite is dropped atcontrolled rates to an exfoliation tray 12 inside the microwave cavity 2using the chute 13 that is located on the top side 11 of the microwavecavity 2. The exfoliation tray 12 is made of non-conductive materialsthat can withstand high temperature environment. Examples of suchmaterials are fused quartz and alumina. The exfoliation tray 12 issitting on top two support rods 14 made of similar materials. These rods14 extend along the width of the microwave cavity 2 and are fastened byoutside nipples 15 protruded on the sidewalls 16 of the microwave cavity2. This configuration of the exfoliation tray 12 and the support rodscan be best observed in FIG. 4 which is a full cross sectional side viewof the device of FIG. 3 through line B-B.

The exfoliated graphite has a much lower bulk density compared to theintercalated graphite or the expanded graphite. Thus, extracting theexfoliated graphite from the microwave cavity 2 can be done bysuspending the exfoliated graphite in air or nitrogen. The microwavecavity exhaust 17, which is connected to a cyclone (not shown) ,extracts the exfoliated graphite from the microwave cavity 2. To controlthe suspension velocity of the exfoliated graphite and the gas flowwithin the cavity 2, compressed air or nitrogen is flown into themicrowave cavity 2 from the inlets 18, which are found in the bottompart of the movable wall 4. By flowing compressed air or nitrogen on thebottom part of the microwave cavity 2, the pressure within that regionof the microwave cavity 2 drops. The introduction of pressuredifferential creates a macroscopic drift of air from the top part of themicrowave cavity 2 into the bottom part of the microwave cavity 2. Thus,the suspended exfoliated graphite flows downward. The microwave cavity 2is tilted at an angle to minimize the exfoliated graphite depositing onthe microwave cavity 2 bottom wall 19 and to maximize the extraction ofthe exfoliated graphite using the microwave cavity exhaust 17.

The waveguide is pressurized to prevent the exfoliated graphite fromflowing into the waveguide transmission line. The back area of themovable wall 4 is also pressurized to prevent the exfoliated graphitefrom obstructing the fingerstock gaskets 7.

The chute 13 and the exhaust 17 of the microwave cavity 2 have a gridlike structure as seen in FIGS. 2, and 4. This structure is built toprevent microwave radiation. Essentially, the grid can be seen as agroup of small waveguides of which their dimensions are small enoughthat their cutoff frequency is much higher than the operating frequency.Thus, the microwave is attenuated through these waveguides or grids.

The bottom 19, front 20, and part of the sides 16 of the microwavecavity 2 are enclosed with cooling jacket 21 to maintain the microwavecavity wall temperature low, such as below 120 F. The cooling agent isin liquid form such as water or ethylene glycol.

Viewing windows 22 can be built on the side of the microwave cavity 2 toenable one to observe the exfoliation process. The view of the viewingwindows 22 can best be observed in FIG. 3, which is a full side view ofthe device 1 of this invention. The viewing windows 22 are made ofperforated metal with holes diameter much less than 1/10 of thewavelength of the operating microwave frequency. The perforated metalsare covered with glass or any other optically transparent medium.

What is claimed is:
 1. A method of exfoliating a nanomaterial said method comprising: A. providing a nanomaterial; B. placing the nanomaterial in a hollow cavity of a device, said device being a single mode microwave device, said single mode microwave device comprising: a stainless steel housing comprised of a top, a bottom, two side walls, a front, and a moveable back wall, all forming a hollow cavity; said moveable wall having a back surface, and fixedly attached to said back surface, a control rod; contained within said housing, an exfoliation tray, said exfoliation tray supported by at least two support rods, which rods are supported at predetermined levels by the side walls, said exfoliation tray being electrically non-conducting; said bottom, front wall, and side walls having a cooling jacket mounted on outside surfaces; an adapter surmounting the top of the housing; said moveable back wall having at least one gas inlet port therethrough; said bottom of said housing containing therein, an exhaust port; fingerstock gaskets attached to the edges of the moveable back wall to retain electrical contact between the moveable back wall and the housing; a microwave generator to deliver microwave energy to the hollow cavity using a rectangular waveguide that is connected to said adapter; C. moving the moveable back wall of the device such that the nanomaterial is in resonance with the microwave frequency inside the cavity; D. heating the cavity until the nanomaterial is exfoliated; E. removing the yield of exfoliated nanomaterial from the hollow cavity.
 2. A method for preparing exfoliated graphene, said method comprising: A. using a single mode microwave device, said device being a single mode microwave device, said single mode microwave device comprising: a stainless steel housing comprised of a top, a bottom, two sidewalls, a front, and a moveable back wall, all forming a hollow cavity; said moveable wall having a back surface, and fixedly attached to said back surface, a control rod; contained within said housing, an exfoliation tray, said exfoliation tray supported by at least two support rods, which rods are supported at predetermined levels by the side walls, said exfoliation tray being electrically non-conducting; said bottom, front wall, and side walls having a cooling jacket mounted on outside surfaces; an adapter surmounting the top of the housing; said moveable back wall having at least one gas inlet port therethrough; said bottom of said housing containing therein, an exhaust port; fingerstock gaskets attached to the edges of the moveable back wall to retain electrical contact between the moveable back wall and the housing; a microwave generator to deliver microwave energy to the hollow cavity using a rectangular waveguide that is connected to said adapter; B. while the single mode microwave device is operating, delivering intercalated graphite at a controlled rate to the exfoliation tray through the delivery chute while controlling the microwave frequencies at a frequency selected from the group consisting of: a. 915 MHz and, b. 2.45 GHz.
 3. A product prepared by the method as claimed in claim
 1. 4. Exfoliated graphite produced by the method of claim 1 having a bulk density of 0.012-0.035 grams/cm³.
 5. Exfoliated graphite produced by the method of claim 1 having BET ranges from 50-300 m²/gr.
 6. The method of providing exfoliated graphite as claimed in claim 1 wherein the yield ranges from 0.5 to 10.0 Kg/hr.
 7. The method as claimed in claim 6 wherein the yield of exfoliated graphite ranges from 1 to 4 Kg/hr.
 8. The method as claimed in claim 1, wherein, in addition, there is a gas flow rate ranging from 1 to 10 CFM through the at least one gas inlet port.
 9. The method as claimed in claim 1, wherein a residence time ranges from less than a second up to several seconds. 